(1) Field of the Invention
The present invention relates to an image forming apparatus that prevents color deviations from occurring to a reproduced image.
(2) Description of Related Art
In general, when a color image is formed using a full-color image forming apparatus, data of an image to be formed is divided into reproduction colors cyan, magenta, yellow, and black. A toner image is first formed for each reproduction color, and then a color image is formed by transferring the toner images onto, for example, a recording sheet. If image forming positions for the colors are deviated from one another, color deviations occur to a reproduced color image. This results in an extremely deteriorated image quality. Hereinafter, the reproduction colors are respectively referred to as C, M, Y, and K and components related to these colors are assigned numerals with a corresponding C, M, Y, or K.
As one example of a conventional full-color image forming apparatus, there are so-called "tandem-type" image forming apparatuses. A tandem-type image forming apparatus has image forming units corresponding to reproduction colors set in line in a direction in which a recording sheet is transported on a transfer belt (this direction is referred to as the "transporting direction" hereinafter). The apparatus forms a color image by transferring the toner images formed in the reproduction colors while adjusting a timing of image formation for each reproduction color. On this account, color deviations are likely to happen when a color image is formed using the "tandem-type" image forming apparatus. One of the biggest problems is how such color deviations can be reduced.
The color deviations may be caused when image forming positions for the four colors are uniformly deviated one another. Other than this, color deviations may be caused when scanning lines formed on photosensitive drums by laser beams are skewed or bowed, or when a scaling factor varies, and they may differently occur for each image forming unit. These cases take place due to the following reasons. One reason is non-uniformity of refraction characteristics of scanning lenses provided for an optical scanning system that scans the laser beams across the photosensitive drums of the image forming units. Another reason is inadequate adjustments to angles of redirecting mirrors. As a further reason, the color deviation is ascribable to displacements in the image forming positions that are caused by expansions of position setting members in consequence of changes in temperature.
To address these problems causing color deviations, various techniques for correcting the image forming positions have been suggested for each problem. As one example, suppose that the image forming positions are deviated from one another on the whole in both the main scanning and the sub-scanning directions. In this case, the problem can be avoided by changing an exposure timing and an image formation timing for each image forming unit in the main scanning and sub-scanning directions. However, if only these timings are controlled, color deviations caused from skews, bows, or variations in the scaling factors cannot be avoided.
For color deviations caused from the variations in the scaling factor in the main scanning direction, appropriate methods have been suggested. By means of one method example, positions of mirrors provided in a print head are changed so that an exposure distance is also changed and, as a result, the scaling factor is controlled. As another method example, scaling correction is performed in the main scanning direction by modulating laser clocks that are used for exposure. This method is disclosed in Japanese Laid-Open Patent Application No. 63-64473, for example.
For color deviations caused from skews in the sub-scanning direction that are different for each color, specific methods have been practically used. Specifically, using one of the methods, the image formation positions are mechanically corrected by actually adjusting inclination of an image forming unit, or a photosensitive drum in particular, for each color. Using another one of the methods, the relative deviation between two points is detected in the main scanning direction, and in accordance with the detection result, the mirrors in the print head are moved so that the image formation positions are corrected.
However, using the method of changing the positions of the mirrors to avoid the color deviations caused from the variations in the scaling factor in the main scanning direction, the construction of the image forming apparatus increases in complexity and is also subject to adverse affects of mechanical vibrations or the like. For this reason, greater rigidity should be provided for the apparatus to withstand vibrations. This results in increases in both size and costs for manufacturing the image forming apparatuses. When using the method of correcting the scaling factor in the main scanning direction through modulating the laser clocks, the modulation should be performed at a high speed. Thus, a CPU with a high processing speed needs to be used, and therefore, costs would be increased. Here, using either of these methods, it is difficult to change the amount of adjustment in one scanning line in terms of stability.
Meanwhile, using the method of actually adjusting the inclination of the photosensitive drums to avoid the skews in the sub-scanning direction, the adjustments need to be performed when the photosensitive drums are replaced. Therefore, it is difficult to automate the adjustments. By means of the method of moving the mirrors in the print head, the apparatus becomes subject to mechanical vibrations, and therefore, the apparatus needs to be increased in size. Here, using either of these methods, it is impossible to avoid the color deviations caused from the bows even though possible to avoid the color deviations caused from the skews.